Reflector antennas are widely used in the millimeter-wave region. They are typically single-beam antennas of moderate or high directivity gain for communication, radar and sensing, and monopulse antennas for tracking and guidance due to their large surface. Most of the beam scanning antennas, based on the principles of the reflector antennas, in commercial use today are mechanically controlled and are thereby capable of mechanical scan. This has a number of disadvantages including: limited beam scanning speed as well as limited lifetime, reliability and maintainability of the mechanical components such as motors and gears.
Microwave terrestrial and satellite communications systems are rapidly being greater than deployed to serve communications needs. In these systems, to ensure a radio communication link between a fixed station on the ground or on a satellite and a mobile station such as an automobile or airplane, antenna systems with scanning beams have been put into practical use. A scanning beam antenna is one that can change its receiving/transmission direction, usually for the purpose of maintaining a radio link, e.g. to a tower or satellite, as a mobile terminal is moving and changing direction. Another application of a scanning beam antenna is in a point-to-multipoint terrestrial link where the beams of a hub antenna or remote antenna must be pointed at different locations on a dynamic basis.
Electronically scanned antennas are becoming more important with the need for higher speed data, voice and video communications through geosynchronous earth orbit (GEO), medium earth orbit (MEO) and low earth orbit (LEO) satellite communication systems and point-to-point and point-to-multipoint microwave terrestrial communication systems. Additionally, new applications such as automobile radar for collision avoidance can make use of antennas with electronically controlled beam directions.
Phased array antennas are well known to provide such electronically scanned beams and could be an attractive alternative to mechanically tracking antennas because they have the features of high beam scanning (tracking) speed and low physical profile. Furthermore, phased array antennas can provide multiple beams so that multiple signals of interest can be tracked simultaneously, with no antenna movement. Phased array antennas are capable of steering transmission and reception beams over a field of view. A phased array may be used to point a fixed radiation pattern, or to scan rapidly in azimuth and/or elevation. Beam scanning in a volume array is accomplished by connecting a phase shifter to every element and compensating for phase differences between the elements for a desired scan direction. The directivity of a phased array antenna is largely determined by the number of antenna elements in the phased array. Therefore, generally the phased array antennas are composed of hundreds or even thousands elements increasing the complexity and the cost of such antennas.
Adding a reflector, such as a parabolic reflector, to the phased array antenna can increase the directivity of the antenna without increasing the number of phased array elements. Most reflector antennas are focused systems that use a single feed aligned to the focal point of the reflector or reflector system. The focused system uses a focused antenna where the reflector(s) serves to focus the energy incident on the main reflector at a single point. When an array feed is used with a focused reflector system, feed array elements that are not on the focal point produce beams that have significant phase error, since they are not focused, resulting in distorted beam shapes and reduced beam gain. Moreover, the electronic scanning capability of the phased array fed reflector antenna is limited to about ±10 beamwidth scan for a given frequency (for example for high gain antenna until about 2° angle) (see for example Mrstik A. V., & Smith, P. G., “Scanning Capabilities of Large Parabolic Cylinder Reflector Antennas with Phased-Array Feeds” IEEE Trans. Antennas Propagat., vol. AP-29, May 1981).
Another technique is to use a very long focal length reflector to reduce the defocusing effects with scan. In this technique, the feed element displacement from the focal point required to scan the beam is proportional to the focal length.
In addition to having a large aperture, many antennas preferably have agile scan capability, which is the ability to rapidly (i.e., electronically, instead of mechanically) scan a region over a wide angular range. In a phased array antenna, a set of amplitude and phase control electronics drive each radiating element. The control electronics are typically quite flexible and allow a phased array antenna to achieve an enormous angular range. For example, a phased array antenna may have an angular range up to about ±70 degrees. Unfortunately, as the aperture size of a phased array antenna increases, the amount of radiating elements and associated control electronics drastically increases, with a concomitant increase in power consumption, thermal dissipation and weight. The complexity of the structural design and the deployment also increase drastically. In other words, large aperture phased array antennas are impractical from economic and engineering standpoints. The presently used phased array antennas are too expensive for most commercial applications. Their use has been generally limited to relatively small quantities of specialized and expensive systems such as military, aircraft, and space systems. Typically, phased arrays employ hundreds or thousands of radiating elements and a correspondingly high number of phase shift elements. Their cost is proportional to the number of elements and the number of active electronic devices such as amplifiers and phase shifters.